Gas discharge lamps utilize electronic ballasts for converting an AC line voltage into a high frequency current for powering the gas discharge lamps. Instant start ballasts typically supply power to several lamps in a fixture. The instant start ballast is frequently used for lamp starting without preheating the lamp filaments. For example, the industry standard, instant start electronic ballast for multiple T8 lamps employs a current fed parallel resonance inverter. Since this inverter is a voltage source rather than a current source, each of these lamps is connected to the inverter output via a boost capacitor. A difference between a current fed half bridge resonance inverter and a voltage fed series resonance half bridge inverter is that in the current fed inverter maximum voltage across switching transistors is more than twice as high as the voltage fed inverter. A half bridge current fed ballast inverter requires high voltage transistors (1100V and higher), whereas in a half bridge voltage fed series resonant inverter the maximum transistor voltage is much lower, i.e., it is equal to the DC bus voltage (430-440V). Voltage fed resonant inverters tend to be more efficient than current fed resonant inverters because voltage fed inverters utilize MOSFETS in a Zero Voltage Switching (ZVS) mode. In addition, the lamp current generated by voltage fed series resonant inverters is almost sinusoidal. It provides longer lamp life than a current fed inverter. Also, voltage fed series resonance inverters can be built without an output power transformer.
To take advantage of voltage fed inverters, multi-lamp ballasts sometimes are provided with several identical resonant tanks, each coupled to a single discharge lamp. For example, U.S. Pat. No. 7,372,215 issued to Sekine et al. discloses a multi-parallel lamp ballast with a single inverter and multiple resonant tanks. In addition to complexity, the above ballast needs to be restarted after replacing a lamp. It is provided with lamp out/in sensing to activate the restart. Patent Application 2007/0176564 issued to Nerone at al. discloses a multi-lamp application of a voltage fed self generated inverter having a regulated output voltage. This inverter is provided with output voltage clamping means since its control does not have enough resolution to limit this voltage at no load. Also, it has a number of multi-winding magnetic components which affect ballast cost.
One challenge in designing a multi-lamp series resonant ballast is to control both the wide range of load variations and the need for sufficient start up voltage. A few of such series resonant ballasts for powering multi-parallel lamps are known. For example, U.S. Pat. No. 6,362,575 issued to Chang et al. discloses a control circuit for a four lamp transformerless series resonance inverter with regulated output voltage. Four boost capacitors, each connected in series with a lamp, are used for ballasting gas discharge lamps. The ballast senses the number of lamps connected by monitoring the current via lamp filaments and generates reference voltages according to number of lamps connected to the ballast. The above approach requires additional wiring between the ballast and the lamps. U.S. Pat. No. 7,352,139 issued to Ribarich et al. discloses a static feedback control circuit for a multi-lamp series resonant inverter with a control IC utilizing a voltage control oscillator (VCO) for frequency control. Since VCO oscillations are not phase locked with resonant load oscillations, the VCO cannot follow changes in resonant load fast enough and may not always oscillate above the resonant frequency. According to the above patent application, the VCO integrates its input signal, causing a delay in dynamic frequency response. During transients in the resonant load (a gas discharge lamp may significantly change its resistance in few microseconds) or lamp removal, this delay can cause temporarily hard switching in the inverter MOSFETS and damage the inverter. ICs with adaptive ZVS (IR 2520D and other similar adaptive circuits) do not eliminate the cross conduction phenomena in switching transistors during unexpected transients in inverter load. U.S. Pat. No. 7,030,570 assigned to Osram Sylvania discloses a series resonant inverter single lamp operation in which hard switching is avoided during load transients.
Nevertheless, there is a need for a ballast control circuit and method aimed at multi-lamp instant start applications. Parallel connected lamps are preferable in multi-lamp series resonant ballast since the light in not interrupted when replacing lamps in a fixture. Existing control methods for multi-lamp inverters (0 load) are based on the concept that the resonant inverter voltage is regulated and ballasting of lamps is achieved with series capacitors. In one embodiment, the present invention provides a method and control circuit for parallel multi-lamp instant start operations that utilize the ballasting features of both resonant inverters and series capacitors.